1. Field of the Invention
The invention generally relates to a control unit, sensing device and a method, and particularly to a control unit, sensing device for a capacitive touch panel and a method therefor.
2. Description of Related Art
For a long time, the main means of input for electronic devices have been keyboard and mouse. Touch panels which receive the input information by using user's hands or touch pens to touch sensing areas on their panels have drawn attention and have been put into practice already. There are sensing devices underneath the touch panels corresponding to the sensing areas. Touch panels are categorized by the ways touch input is recognized, including resistive touch panels, capacitive touch panels, sound-wave touch panels, optical touch panels, and electromagnetic touch panels
In capacitive touch panels having capacitive point-to-touch sensing means for example, all currents passing through a particular touch input location are sensed to determine the location of the touch point by means of their correlation.
FIG. 1 is a perspective view of a conventional capacitive touch panel. As shown in FIG. 1, a capacitive touch panel 10 consists of a transparent substrate 11, an electric conductive film 12, an electrode pattern 13 and an insulating hard layer 14. The transparent substrate 11 can be made of glass, for example. The electrode pattern 13 is formed along the periphery of the touch panel 10 for compensating curve distribution of an electric field for the conductive film 12 at work.
FIG. 2 is a schematic view of a location determining circuit for a conventional capacitive touch panel.
Four corners of the touch panel 10 are respectively connected to external wires A, B, C and D, and respectively receive alternative sensing signals AC1, AC2, AC3 and A4 to provide location data for a touch point P on the touch panel 10.
In operation, the alternative sensing signals AC1, AC2, AC3 and AC4 are alternative square waves or sinusoidal wave voltage signals. Currents I1, I2, I3 and I4 respectively pass through the corresponding external wires A, B, C and D. The coordinates of the touch point P can be calculated based on the following formula by respectively measuring the current changes ΔI1, ΔI2, ΔI3 and ΔI4 for the corresponding external wire A, B, C and D before and after the touch point appears:x=(ΔI3+ΔI4−ΔI1−ΔI2)/(ΔI1+ΔI2+ΔI3+ΔI4)  (1)y=(ΔI1+ΔI4−ΔI3−ΔI2)/(ΔI1+ΔI2+ΔI3+ΔI4)  (2).
The current change ΔI1, ΔI2, ΔI3 and ΔI4 can be obtained in the manner as shown in FIG. 2. The current change ΔI1 is obtained by measuring the current I1 by using a current measuring circuit 15, transmitting the current I1 to a filter circuit 16, reducing noise, integrating the noise-reduced current I1 by using an integrating circuit 17, transmitting the integrated current I1 to a controller 18, converting the analogue current signal into a digital signal by using an analogue/digital conversion circuit 180 in the controller 18, and calculating the current change by using a location determining unit 182 in the controller 18. Similarly, ΔI2, ΔI3 and ΔI4 can be obtained in turns. The current change ΔI1, ΔI2, ΔI3 and ΔI4 are processed in turns with the same analogue/digital conversion circuit 180 and calculated based on the above formula to obtain the X and Y coordinates of the touch point.
However, there can be a time difference between the calculated coordinate and the real coordinate of the touch point. Additionally, noise generated by the filter circuit 16 and the integrating circuit 17 or external noise can cause errors in the determined coordinates. Furthermore, the configuration as shown in FIG. 2 increases production costs. For example, it requires a large amount of current measurement circuits 15, filter circuits 16 and integrating circuits.